The co-sintering or firing of laminated ceramic tapes is a well-known module manufacturing method in the microelectronics industry. The term “low-temperature co-fired ceramic (LTCC)” refers to a technology for forming multilayered ceramic circuits. In this approach, a tape is formed from glass and ceramic powders uniformly dispersed in an organic binder. Typically, two or more layers of this tape are laminated together to form a circuit. To form electrical connections from one layer of tape to the next, via holes are punched through the tape and filled with a thick-film conductor paste, for example as described in U.S. Pat. No. 4,654,095 of Steinberg.
In the next step, thick-film pastes (dispersions of metallic, ceramic or glass powders in volatilizable organic vehicles) that will form components of electronic circuits, such as conductors or resistors, are then screen-printed onto the tape. After all of the layers of tape necessary to form the completed circuit have been prepared, the pieces of tape are aligned to ensure that the via connections from one layer will make contact with conductor traces or via holes on the next. The layers of tape are then laminated with a combination of heat and pressure to form a single green body, i.e., a structure that is held together by organic binders, such as polyvinyl butyral or acrylate materials.
In order to form the final ceramic body, the green body is fired in a firing profile that typically reaches a peak temperature of about 850° C. to 900° C. before returning to ambient temperature. In a range of temperatures between about 350° C. and 450° C., the organic binders that give the green body strength are volatilized or burned out. To give the volatilized gases sufficient time to escape, the ramp rate (change in temperature per unit time) of the profile is often reduced in this temperature range.
Above the burnout temperature, the ramp rate of the firing profile is increased and the part is heated until reaching the peak firing temperature. The LTCC tape typically contains a significant amount of a glass with which a glass softening point Tg is associated. The glass and ceramic powders will begin to sinter into a dense body when the temperature is above the softening point of the LTCC glass, so the peak firing temperature of the tape is typically 100° C. to 200° C. above the Tg. The thick-film conductor and resistor materials used in the circuit body will undergo a similar metamorphosis from organically bound powders into dense sintered structures. After allowing the parts to remain at the peak firing time to reach an adequately dense body, the parts are cooled to room temperature.
Manufacturing of LTCC tapes is typically performed using tape casting techniques, such as those described in U.S. Pat. No. 5,821,181 of Ursula, et al. In this method, ceramic slurry (a mixture of the inorganic and organic components of the tape before drying) is deposited on top of a polyester film or carrier using a doctor blade. One disadvantage of using tape casting techniques for tape manufacturing is the difficulty of thickness control as the tape becomes thinner and thinner. More specifically, thickness, accuracy and variance become uncertain when casting under 2 mils (50 microns), a measurement which refers to the gap between the blade and the backing as the wet slurry passes through. Therefore, control of the layer thickness, especially of inner layers, becomes difficult and often inaccurate.
While accurate casting of individual layers is achievable, the method described in U.S. Pat. No. 5,102,720 for drying the tapes individually and subsequently laminating them together is uneconomical. Thus, methods which involve drying individual layers and lamination with heat and pressure, or casting a subsequent layer on top of a dry layer, not only introduce significant costs to the manufacturing process, but also limit product yields.
Other manufacturing methods include dipping a moving carrier film in a slurry to create a meniscus between the carrier film and the slurry. However, the meniscus created by capillary forces between the wet organic binder and the film causes it to stick to the surface of the polyester film. As in other methods, drying one layer at a time and then casting a wet layer on top of a dry layer or subsequent heat lamination are needed. Because of the disadvantages with known methods for manufacturing LTCC tapes, there remains a need in the art for an improved, economical method for fabricating LTCC tapes which will maximize product yield and permit tight control of layer thickness.
The LTCC technology has advanced beyond the microelectronic circuit industry and is currently in use for a variety of applications. One important attribute of LTCC is the ability to create three-dimensional structures using multiple layers of tape. The biomedical device industry, for example, uses LTCC for the manufacturing of cavities and channels for moving part pumps used in in-situ drug delivery systems. Biological test modules have been realized which facilitate the automatic testing of biological and chemical materials.
In the telecommunications industry, there is a need for integrated opto-electronic modules. LTCC offers the simplicity of being able to co-sinter optical fibers together with the driving electronics. The co-firing of meso-scale structures containing metallization, cavities, vias, and channels is thus an appealing feature of LTCC.
LTCC meso-systems are small packages capable of handling at least two media, such as electricity and fluids, by means of sensors, actuators, interconnection, control and/or signal processing. Miniaturization is one of the biggest drivers of this technology, thus allowing systems in package (SIP), in which several components are inserted into a monolith.
An attractive feature of LTCC tapes is the possibility for making cavities for the placement of integrated circuits within. For example, as shown in FIG. 1A, a single electronic module 15 contains a cavity 15A, a metallic via 15B, and a metallic line trace 15C on the surface of the ceramic monolith. FIG 1B shows a panel area 17 which contains an array of microelectronic modules 15. The panel 17 is typically formed and processed as a whole and then cut into individual modules 15.
Cavities allow a module to retain a low profile, while a lid may be placed on top for hermeticity. However, during surface or sacrificial constrained sintering, as explained below, the cavity walls exhibit a phenomenon called necking, a vertical curvature from the top surface interface to the bottom of the fired substrate surface. During sintering of sacrificially constrained structures, there is a stress distribution due to the shear and in-plane tensile stresses from top to bottom. It has been shown that stresses are significantly higher at the constrained interface. Moving along the z-axis towards the middle of the fired substrate, there are fewer constraining forces that counteract the in-plane tensile stresses. Therefore, there is significantly more densification in the middle of the monolith, which causes the vertical curvature. Furthermore, as a consequence of the higher stress distribution at the interface, delamination or buckling is usually present. The aforementioned properties are undesirable, especially when constructing cavities or other precision features in the ceramic structures.
Despite the numerous applications of LTCC technology, the LTCC process has several disadvantages. First, there are significant changes in the dimensions of the ceramic monolithic structure during sintering. More specifically, when the constituent powders of the LTCC structure densify during traditional unconstrained or free sintering, shrinkage occurs in all dimensions. Typically, the shrinkage of the tape across its width or length (the x- or y-directions) will be nearly identical and only slightly different from the shrinkage through the thickness of stack-up of tape layers (the z-direction). Usually, the dimensions of the structure after firing will be about 84% to 87% of the size in the unfired green state. This change and the associated variations result in several disadvantages to the use of conventional LTCC technology.
First, the shrinkage in the x-y directions, Sxy, requires that the area of green tape used to make an unconstrained circuit be a factor of 1/(1−Sxy)2 greater than the ultimate fired area. Consequently, the green tape area used to make a free sintered circuit should be about 25% to 40% larger than the final circuit.
A second disadvantage of unconstrained sintering is the loss of geometric precision that occurs in free sintered circuits. This loss of precision limits the ability to produce large numbers of microelectronic single parts or individual modules in a single LTCC panel size, the size of a large array of modules built in a single LTCC substrate. Specifically, this size is limited because of the shrinkage in the planar (x and y) directions, usually 10-15%, and its variation, typically about ±0.2% to ±0.4%. This variation becomes increasingly problematic as the size of the LTCC circuit or the devices which will be placed on the circuit increases, or as the interconnect pitch (the space between connections on a package) decreases. For example, if the dimensions of a panel size are 8 in×8 in (203 mm×203 mm), such a variation would result in a positional uncertainty of about ±16 mils (41 μm).
During firing, the shrinkage uncertainty of the LTCC causes the external features to vary with respect to the intended nominal position. Artworks used for post-firing processes, such as the printing of post-fired conductors or resistors, or for printing solder on conductors, are based on the intended nominal position. Excessive distance between the actual fired position of a circuit feature and the nominal position can cause circuit failures if, for example, there is failure to make adequate electrical contact, which may result from lack of via connections or misalignments between layers due to shrinkage uncertainty. Alternatively, although artwork features may be enlarged to allow for such shrinkage variation, decreased circuit density may result.
The aforementioned problems, which limit the ability to co-fire embedded components and/or to create distortion-free cavities within a ceramic monolith, have driven the microelectronics industry to resort to constraining practices in order to reduce the dimensional uncertainty of ceramic panels during the firing process. A variety of such constraining methods have been used in the industry to try to adapt and circumvent the shrinkage problem.
For example, pressure-assisted sintering and the application of external loads on top of ceramic tape modules are described in U.S. Pat. No. 4,340,436. The use of mechanical clamping on the periphery of a ceramic panel to contain its x-y dimensions is discussed in European Patent No. 0 243 858.
These types of approaches present several potential problems and disadvantages to the manufacturer. Because the presence of the platen may cause functional defects in any conductors or resistors which are in direct contact with the surface of the LTCC, the platen contact geometry must be carefully controlled and aligned with the green tape. Use of mechanical clamping techniques may require different platen designs for different circuits. Finally, a separate platen must be used for each constrained structure being fired in a batch.
Alternatively, the use of porous contact sheets attached to the LTCC panels that are easily removed after sintering is described in U.S. Pat. No. 6,139,666. Additionally, as described in U.S. Pat. No. 6,205,032 and U.S. Patent Application Publication No. 2001/0018797, the use of a constraining ceramic core that constrains the attached layers using subsequent firings has been attempted.
A further technique for constraining the x-y geometry of LTCC circuits involves laminating sacrificial constraining tape layers onto the top and bottom surfaces of the LTCC circuit body. This technique has been described, for example, in U.S. Pat. Nos. 5,085,720; 5,254,191; 5,383,474; and 5,474,741, all of Mikeska, et al. The sacrificial tape layers are formed from porous, high temperature refractory ceramic powder that by itself will not sinter during the LTCC firing process. Since the sacrificial tape does not sinter and densify during the firing profile, it maintains the geometry of its green state.
However, in order for the sacrificial refractory tape to constrain the x-y geometry of underlying LTCC tape, at least two conditions should be met. First, there must be sufficient friction between the two tape materials to mechanically link the materials. Second, glassy components of the LTCC tape that could dissolve the refractory component of the sacrificial tape during the LTCC firing profile, thus allowing it to sinter and densify, must not saturate the sacrificial tape layer.
All of the aforementioned external constraint approaches have significant drawbacks. For example, pressure-assisted sintering and peripheral constraining require special adaptation of the furnace or the need for external equipment to mechanically prevent shrinkage of the ceramics. Other methods require the creation of refractory ceramic porous molds to form the tape for cavities.
Finally, several potential problems exist for manufacturers using sacrificial tape processes. After firing, the sacrificial tape layer must be removed from the circuit body sufficiently completely to not interfere with subsequent manufacturing processes, but not so aggressively as to damage the remaining LTCC body. Like the platen of the mechanical clamping technique, the sacrificial tape may be incompatible with conductors or resistors that may be placed on the surface of the LTCC circuit body. Therefore, these surface features must be printed and fired after removal of the sacrificial layer, which increases the number of processing steps on the manufacturing line and also results in increased cost of successive firings (furnace costs). From the standpoint of process yield and process simplicity, it would have been preferable to print these features on green tape and co-fire them with the rest of the circuit body. Further, because the sacrificial tape has virtually no mechanical strength after firing, it cannot be incorporated into the body of the LTCC circuit. This limits the thickness of bodies that can be constrained with this method, as the degree of constraint deteriorates with an increase in the distance from the constraining layer. Finally, contact sheets of refractory ceramic sacrificial tape have the potential for surface contamination of the LTCC tape, and the removal or dusting and waste of the sacrificial layer contribute to and reflect on the individual module cost.
There remains a need in the art for a method of constrained sintering for low temperature co-fired ceramic that does not have the drawbacks and limitations of currently employed methods. Such a technology must ensure that x-y dimensions established during via punching and printing are maintained during firing. Additionally, the method should reduce dimensional uncertainty in ceramic parts and eliminate many of the circuit development and manufacturing steps necessary to avoid dimensional errors and misregistration.